A collimator marketed by Nomos Corporation; US., consist of two rows of binary collimators called vanes. Each vane can be in or out and typically covers an area of 1 cm by 1 cm when projected to the patient. These two rows cover an area of 20 cm by 2 cm. Usually the collimator is rotated around the patient and the vanes moved in and out of the radiation field in order to modulate the intensity of the radiation and create the desired dose distribution inside the patient. The system is illustrated in U.S. Pat. No. 5,596,619.
At the completion of the irradiation, one longitudinal slice of the patient has been irradiated and the patient is moved longitudinally (indexed) to the position for the next slice. This motion needs to be very precise as inaccuracies lead to over or under dosages in the tumour and might jeopardise the patient's outcome.